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Subject: Re: [boost] [string] Yet another Unicode string class
From: Jeff Flinn (TriumphSprint2000_at_[hidden])
Date: 2011-02-15 07:35:01

Mathias Gaunard wrote:
> On 15/02/2011 00:12, Howard Hinnant wrote:
>> On Feb 14, 2011, at 5:55 PM, John Bytheway wrote:
>>> On 14/02/11 19:08, Scott McMurray wrote:
>>>> On Mon, Feb 14, 2011 at 01:53, Mathias Gaunard
>>>> <mathias.gaunard_at_[hidden]> wrote:
>>>>> SBO makes moving a string costly.
>>>> How bug a buffer does SBO usually use?
>>> The libc++ string uses 23 bytes (on a 64-bit architecture) by squishing
>>> the length into a single byte for short strings.
>>> I don't see how that makes moves costly, though. The move constructor
>>> for this string simply copies the bytes and zeroes out the source; it
>>> doesn't even need to branch based on whether it's a long or short
>>> string. Perhaps the concern is that such techniques are not defined
>>> behaviour? Or that the use of unions might confuse the optimizer?
>> It isn't so much the size of the buffer that matters, but rather the
>> total number of words that need to be copied or otherwise manipulated
>> (e.g. zeroed). The libc++ string is 3 words (on 32 and 64 bit) and
>> the cost of the move is proportional to 3. If the libc++ string
>> needed to move (for example) 6 words, then its move constructor would
>> be twice as expensive, though still cheap compared to a copy of a long
>> string.
> I didn't know it was just 3 words, that's still very cheap.
> Using memcpy doesn't break the strict aliasing guarantee, so that's legal.
> It could be even cheaper if it could be copied using SIMD instructions,
> but then that would put restrictions on the alignment of string, which
> one might not want.

This caught my eye. A couple of years ago I looked at MMX/SIMD - tested
an example off of the codeguru website which out of the box showed
MMX/SIMD was faster. I unrolled the loop of the non-MMX/SIMD code using
duff's device and saw no difference in performance with or without
MMX/SIMD. I'm just wondering in your experience is there a real benefit
of MMX/SIMD even discounting the alignment issue.

Thanks, Jeff

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